Field of the Invention
The present invention provides amidic polymers, which exhibit shale swelling inhibitor activity having improved biodegradability. The amidic polymers of the invention may be employed in a wide variety of compositions, particularly in subterranean drilling operations.
Description of Related Art
Subterranean drilling operations are procedures conducted below the surface of the earth from a formation such as in the course of recovering oil, gas, and other substances. Typically, rotary subterranean drilling operations involve attaching a drill bit on a lower end of a drill string to form a drilling tool and rotating the drill bit along with the drill string into a subterranean formation to create a well bore through which subsurface formation fluids may be recovered. During subterranean drilling operations, subterranean well bores may encounter strata having reactive shales. Reactive shales are materials like clays (bentonite) and other subterranean materials that swell when exposed to water. Reactive shales can cause problems during drilling because of their tendency to degrade when exposed to aqueous media such as aqueous-based drilling fluids. This degradation can result in undesirable drilling conditions because degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until the cuttings can be removed by solids control equipment located at the surface. Degradation of drilled cuttings prior to their removal at the surface greatly prolongs drilling time because the shale particles traveling up the well bore break up into smaller particles. These smaller particles increasingly expose new surface area of the shale to the drilling fluid, which leads to still further absorption of water and further degradation.
Consequently, subterranean drilling operations generally employ drilling fluids to inhibit shale swelling and improve environmental performance. Drilling fluids used in such operations may be fluids (gaseous or liquid), mixtures of fluids, and solids (solid suspensions, emulsions, gases and solids). Drilling fluids are also used to cool the drill bit, lubricate the rotating drill, prevent blowouts, and remove drill cuttings from the well bore.
Shale disintegration may also affect Equivalent Circulating Density (ECD). ECD is the increase in bottom hole pressure due to friction and is normally expressed in pounds per gallon. ECD, when forward circulating, is the apparent fluid density, which results from adding annular friction to the actual fluid density in the well. ECD may be affected by the solids content of the drilling fluid, which may increase if surface solids control equipment cannot remove shale from the drilling fluid. Plastic viscosity (an indicator of size and quantity of solids) is an important parameter that affects drilling rate. Maintenance of appropriate ECD is important in drilling a well bore where a narrow tolerance exists between the weight of the drilling fluid needed to control the formation pressure and the weight of the drilling fluid that will fracture the formation. In such circumstances, minimizing shale degradation may be desirable to control the viscosity of the drilling fluid. Shale degradation may substantially decrease the stability of the well bore, which may cause irregularities in the diameter of the well bore. Shale degradation may decrease the stability of the well bore to such an extent that the well bore collapses. Degradation of the shale also may interrupt circulation of the drilling fluid, cause greater friction between the drill string and the well bore. Accordingly, the complications associated with shale swelling during drilling may substantially increase the time and cost of drilling.
Background material on shale swelling inhibitors include U.S. Pat. Nos. 7,087,554, 7,549,474, and 7,833,945. Further background includes A. V. Reis, et al., J. Org. Chem., 2009, 74, 3750-3757; U.S. Pat. No. 5,037,930; S. H. Kim, et al., Carbohydrate Polymers 40 (1993) 180-190; and Fhilype J., et al., International Journal of Pharmaceutices 335 (2008) 184-194.
Traditional methods for inhibiting shale swelling during drilling have employed oil-based and synthetic-based drilling fluids. However, water-based shale swelling drilling inhibitors are now generally used because they are considered more environmentally friendly. Many shale swelling inhibitors show good inhibition performance in the oilfield but their application may be limited because of poor biodegradability. Accordingly, there is a need for shale swelling inhibitors having improved biodegradability.